diff --git a/src/librustc_mir/transform/add_yk_swt_calls.rs b/src/librustc_mir/transform/add_yk_swt_calls.rs
index 7560d04115d..e7b3a809f45 100644
--- a/src/librustc_mir/transform/add_yk_swt_calls.rs
+++ b/src/librustc_mir/transform/add_yk_swt_calls.rs
@@ -22,8 +22,8 @@ use rustc::hir::map::blocks::FnLikeNode;
 /// The arguments to the calls (crate hash, DefId and block index) identify the position to be
 /// inserted into a trace.
 ///
-/// The transformation works by copying each block and replacing the original with a new block --
-/// its "shadow" -- which calls the trace recorder function, before returning to the copy.
+/// The transformation works by copying each original "user block" and replacing it with a new
+/// block -- its "shadow" -- which calls the trace recorder function, before returning to the copy.
 ///
 /// For example:
 ///
@@ -32,8 +32,8 @@ use rustc::hir::map::blocks::FnLikeNode;
 /// +----+   +----+           +-----+   +-----+   +----+   +-----+   +-----+   +----+
 ///
 /// Where:
-///  * B0 and B1 are blocks in the MIR before transformation.
-///  * B0' and B1' are replacements of B0 and B1 respectively.
+///  * B0 and B1 are "user blocks" in the MIR before the transformation.
+///  * B0' and B1' are "shadow blocks" of B0 and B1 respectively.
 ///  * B2 and B3 are copies of B0 and B1 respectively.
 ///  * 'Rec' is the trace recorder function.
 ///  * The block indices match the indices in the backing vector in the MIR.
@@ -59,7 +59,7 @@ impl MirPass for AddYkSWTCalls {
         let u64_ty = tcx.types.u64;
 
         let mut shadow_blks = Vec::new();
-        let mut copied_blks = Vec::new();
+        let mut user_blks = Vec::new(); // Copies of the blocks we started with.
         let mut new_local_decls = Vec::new();
 
         let num_orig_local_decls = mir.local_decls.len();
@@ -68,8 +68,8 @@ impl MirPass for AddYkSWTCalls {
         for (bb, bb_data) in mir.basic_blocks().iter_enumerated() {
             // Copy the block.
             let new_blk = bb_data.clone();
-            let new_blk_idx = BasicBlock::new(mir.basic_blocks().len() + copied_blks.len());
-            copied_blks.push(new_blk);
+            let new_blk_idx = BasicBlock::new(mir.basic_blocks().len() + user_blks.len());
+            user_blks.push(new_blk);
 
             // Prepare to call the recorder function.
             let ret_val = LocalDecl::new_temp(unit_ty, DUMMY_SP);
@@ -123,7 +123,7 @@ impl MirPass for AddYkSWTCalls {
         }
 
         // Finally, commit our transformations.
-        mir.basic_blocks_mut().extend(copied_blks);
+        mir.basic_blocks_mut().extend(user_blks);
         mir.local_decls.extend(new_local_decls);
         for (bb, bb_data) in shadow_blks.drain(..).enumerate() {
             mir.basic_blocks_mut()[BasicBlock::new(bb)] = bb_data;